The present invention relates to a DA converter incorporated in liquid crystal driving and other devices, and further relates to a liquid crystal driving device incorporating such a DA converter.
DA (digital-to-analogue) converter externally receives a digital signal and convert it to an analogue signal. For example, in a liquid crystal driving device of an active matrix liquid crystal display device, a DA converter is used to convert input signals, which constitute display data, from digital to analogue for an output to a liquid crystal display section. Some DA converters of this kind include an OP-amplifier composed of MOS transistors.
The following description will explain an arrangement of a liquid crystal display device incorporating the aforementioned DA converter, an arrangement of a TFT liquid crystal panel in that liquid crystal display device, a liquid crystal drive waveform of the liquid crystal panel, and an arrangement of a source driver provided to the liquid crystal display device, in reference to FIG. 7 through FIG. 11 which illustrate an arrangement in accordance with the present invention. In reference to FIG. 12 and FIG. 13, the following description will also explain a conventional arrangement of the aforementioned DA converter. Note in the following description that the present invention and the conventional technology share those common arrangements as presented in FIG. 7 through FIG. 11. The foregoing conventional arrangement is described in CMOS Device Handbook, edited by the Editing Committee for CMOS Device Handbook and published by The Nikkan Kogyo Shimbun Ltd. on Sep. 29, 1987.
FIG. 7 constitutes a block diagram showing an arrangement of a TFT (Thin Film Transistor) liquid crystal display device as a typical active matrix liquid crystal display device.
This liquid crystal display device is divided into two parts: a liquid crystal display section and a liquid crystal drive circuit (liquid crystal driving device) for driving the liquid crystal display section. The liquid crystal display section includes a TFT liquid crystal panel 901; the liquid crystal panel 901 includes therein liquid crystal display elements (not shown) and opposite electrodes (common electrodes) 906 (will be mentioned in detail later).
Meanwhile, the liquid crystal drive circuit includes source drivers 902 and gate drivers 902, each driver being built as an IC (Integrated Circuit), a controller 904, and a liquid crystal drive power supply 905. The controller 904 provides display data D and control signals SI to the inputs of the source drivers 902, and control signals S2 to the inputs of the gate drivers 903. Hence, the controller 904 provides vertical synchronized signals to the gate drivers 903 and horizontal synchronized signals to the source drivers 902 and the gate drivers 903.
The externally provided display data is transmitted through the controller 904 to the source drivers 902 as the display data D with its digital form being retained. The source driver 902 time-sequentially latches the incoming display data, and thereafter, converts the display data from digital to analogue in synchronization with the horizontal synchronized signals received from the controller 904. Then, the analogue voltages (half-tone display voltages) obtained from the DA conversion for half-tone display use are transmitted via liquid crystal drive voltage output terminals and source signal lines 1004 (will be mentioned in detail later) to the respective liquid crystal display elements (not shown) in the liquid crystal panel 901.
FIG. 8 shows an arrangement of the liquid crystal panel 901, including pixel electrodes 1001, pixel capacitors 1002, TFTs 1003 as elements for turning on/off voltage application to the pixels, source signal lines 1004, gate signal lines 1005, and opposite electrodes 1006 (equivalent to the opposite electrodes 906 in FIG. 7). In FIG. 8, the encircled area xe2x80x98Axe2x80x99 represents a liquid crystal display element for one pixel. The source drivers 902 couple half-tone display voltages to the source signal lines 1004 according to brightness of the pixels used for a display. The gate drivers 903 couple scan signals to the gate signal lines 1005 so as to sequentially turn on the vertically lined TFTs 1003. Through the TFT 1003 which is in an On state, the voltage in the source signal line 1004 is applied to the pixel electrode 1001 connected to the drain of that TFT 1003, causing accumulation of charges in the pixel capacitor 1002 formed between the pixel electrode 1001 and the opposite electrode 1006. The accumulation of charges alters the optical transmittance of the liquid crystal and realizes a display.
FIG. 9 and FIG. 10 show liquid crystal drive waveforms as examples. 1101 and 1201 each denote a drive waveform of the source driver 902. 1102 and 1202 each denote a drive waveform of the gate driver 903. 1103 and 1203 each denote a potential of the opposite electrode. 1104 and 1204 each denote a voltage waveform of the pixel electrode. The voltage applied across the liquid crystal material is equivalent to the potential difference between the pixel electrode 1001 and the opposite electrode 1006, which is shown as shaded areas in FIG. 9 and FIG. 10. For example, in FIG. 9, the TFT 1003 is turned on when the drive waveform 1102 of the gate driver is in high level, causing the difference between the drive waveform 1101 of the source driver and the potential of the opposite electrode 1103 to be applied to the pixel electrode 1001. Subsequently, the drive waveform 1102 of the gate driver changes to low level, causing the TFT 1003 to change to an Off state. Here, the aforementioned voltage is retained across the pixel due to the presence of the pixel capacitor 1002. The same explanation holds true with the case in FIG. 10. FIG. 9 and FIG. 10 show different voltages being applied across the liquid crystal material: a higher voltage is applied in the case shown in FIG. 9 than in the case shown in FIG. 10. In this manner, a multiple half-tone display is achieved by applying variable analogue voltage across the liquid crystal and thus altering the optical transmittance of the liquid crystal in an analogue manner. The number of analogue voltages available for application across the liquid crystal determines the number of half-tones displayed.
FIG. 11 shows a block diagram of the source driver 902 as an example. Display data, provided externally as digital signals, consist of display data, DR, DG, and DB for R (red), G (green), and B (blue); the display data is temporarily latched by an input latch circuit 1301, and thereafter stored time-sequentially in a sampling memory 1303 based on the operation of a shift register 1302 which receives a start pulse SP and shifts with a clock CK; the whole data is then simultaneously transferred to a hold memory 1304 in accordance with a horizontal synchronized signal (not shown). xe2x80x9cSxe2x80x9d represents cascade outputs. A standard voltage generating circuit 1309 generates standard voltages of differing levels according to a reference voltage VR. The hold memory 1304 transmits the data through a level shifter circuit 1305 to a DA converter circuit (digital to analogue converter circuit) 1306 where the data is converted to analogue voltages based on the standard voltages of differing levels provided by the standard voltage generating circuit 1309. Then, an output circuit 1307 provides outputs as half-tone display voltages that are transmitted through liquid crystal drive voltage output terminals 1308 to liquid crystal display elements (see xe2x80x9cAxe2x80x9d in FIG. 8).
In this manner, the standard voltage generating circuit 1309, the DA converter circuit 1306, and the output circuit 1307 constitute a DA converter. Further, in the liquid crystal display device, a liquid crystal drive circuit is arranged using the DA converter in the aforementioned manner, and as mentioned above, the DA converter converts digital data (display data DR, DG, DB) from digital to analogue for a display on the liquid crystal panel 901 and applies the converted analogue data to liquid crystal display elements.
FIG. 12 and FIG. 13 show an arrangement into details of a DA converter used in a liquid crystal drive circuit identical to the one above to convert display data from digital signals to analogue voltages for outputs. The DA converter is arranged from a standard voltage generating circuit 1401 (equivalent to the standard voltage generating circuit 1309 in FIG. 11), a selector circuit 1402 (equivalent to the DA converter circuit 1306 in FIG. 11), and voltage follower circuit 1403 (equivalent to the output circuit 1307 in FIG. 11). FIGS. 12 and 13 shows, as an example, the arrangement of a DA converter used for a 64 half-tone liquid crystal drive circuit that provides 64 analogue voltage outputs corresponding to the 6-bit digital signals (Bit5 to Bit0). FIG. 13 is an enlarged view of xe2x80x9cAxe2x80x9d encircling V48 to V64 of the standard voltage generating circuit 1401 and the selector circuit 1402 shown in FIG. 12. The circuits in FIG. 12 are arranged by repeating the arrangement pattern shown in FIG. 13.
The standard voltage generating circuit 1401 generates a plurality of standard voltages (64 different voltages in this example) according to the display data provided as digital signals. The selector circuit 1402, arranged from MOS transistor switches, selects one of those standard voltages for output. The arrangement of the switches will be explained later in detail. The voltage follower circuit 1403 provides the voltage selected by the selector circuit 1402 as an output liquid crystal drive signal through a liquid crystal drive voltage output terminal (equivalent to the liquid crystal drive voltage output terminal 1308 in FIG. 11) to a liquid crystal display element.
Normally, a standard voltage generating circuit 1401 is commonly used for a plurality of liquid crystal drive voltage output terminals.
Meanwhile, a selector circuit 1402 and a voltage follower circuit 1403 are provided for each liquid crystal drive voltage output terminal. In the event of a color display, a liquid crystal drive voltage output terminal is used corresponding to each color; in such an event, a selector circuit 1402 and a voltage follower circuit 1403 are provided for each color in a pixel. Accordingly, supposing that the liquid crystal panel 901 includes the total of N pixels, the liquid crystal panel 901 has liquid crystal drive voltage output terminals R1, G1, B1, R2, G2, B2, . . . , RN, GN, and BN, which requires 3N selector circuits 1402 and voltage follower circuits 1403, where R, G, B denote liquid crystal drive voltage output terminals for red, green, and blue respectively and the subscripted n (n=1, 2, . . . , N) denotes the pixels.
The following description will explain the arrangement and operation of a DA converter used for this liquid crystal drive circuit.
The standard voltage generating circuit 1401 includes an arrangement where 64 resistor elements are connected in series, and receives a largest liquid crystal drive voltage V64 and a smallest liquid crystal drive voltage V0 at terminals located at the respective ends. Therefore, 64 voltages (V0 to V63) are available at respective resistor elements, in proportion to the resistance of the resistor elements. Those 64 different voltages generated by the standard voltage generating circuit 1401 are provided for input to the selector circuit 1402.
In the selector circuit 1402, MOS transistor switches are configured to select one of the 64 input voltages for output according to the display data composed of 6-bit digital signals. Specifically, the switches are turned on/off according to each piece of display data composed of 6-bit digital signals (Bit0 to Bit5). Hence, one of the 64 input voltages is selected for output. The following description will explain voltage selection procedures in detail.
In the 6-bit digital signals, Bit5 is MSB, and Bit0 is LSB. The switches are paired in two to form switch pairs. Bit0 is provided with 32 switch pairs (64 switches), while Bit1 is provided with 16 switch pairs (32 switches). As moving up from a certain bit to a next bit, the number of switch pairs decreases by half, down to a single switch pair (two switches) for Bit5. So, the total number of the switch pairs amounts to 1+2+22+23+2425=63 (126 switches).
The two switches composing a switch pair operate so that if the corresponding bit is xe2x80x9c0xe2x80x9d the upper switch (as can be seen in FIG. 12) turns off, and the lower switch turns on. In contrast, if the corresponding bit is xe2x80x9c1xe2x80x9d, the upper switch (as can be seen in FIG. 12) turns on, and the lower switch turns off. For example, referring to the example shown in FIG. 12, (Bit5, Bit4, . . . , Bit0) is xe2x80x9c111111xe2x80x9d, all the upper switches are on and all the lower switches are off, allowing an output voltage V63 to appear at the output terminal of the selector circuit 1401. Further, for example, if (Bit5, Bit4, . . . , Bit0) is xe2x80x9c000001xe2x80x9d, an output voltage V, appears at an output terminal of the selector circuit 1401. The voltage follower circuit 1403 provides a voltage that is identical to the analogue voltage transmitted from the selector circuit 1402, for output via the liquid crystal drive voltage output terminal as a liquid crystal drive signal having a smaller internal resistance.
If the conventional DA converter is used for a liquid crystal driving device of a liquid crystal display device, the number of elements composing the circuit increases sharply as a larger number of half-tones are to be displayed. Taking a 64 half-tone display as an example, 64 resistor elements are required in a standard voltage generating circuit 1401. Plus, 126 switches are required for every pixel to form a selector circuit 1402. In the same manner, when a 256 half-tone display is performed using 8-bit digital signals, 256 resistor elements are required in a standard voltage generating circuit 1401, and 510 switches are required for every pixel to form a selector circuit 1402 that includes 1+2+22+23+ . . . +27=255 switch pairs.
Further, if a color display is to be performed in the manner mentioned above, since there are three colors included (red, green, and blue), the number of switches required triples.
In this manner, the liquid crystal driving device in accordance with conventional technology requires an increasingly large number of circuit elements to display more colors and half-tones; as a result, the liquid crystal driving device, when fabricated as an integrated circuit, inevitably has a larger chip size.
There is a trend in recent years for the liquid crystal display device to include more minute structures and display more half-tones, resulting in increases in the size of circuits in the liquid crystal driving device. Meanwhile, as the liquid crystal display device finds applications in more fields, there are in the market increasingly higher demands for cheaper liquid crystal display devices and stronger needs to reduce manufacturing costs by manufacturing smaller liquid crystal driving devices.
However, as mentioned above, conventional technology requires greatly larger number of circuit arrangement elements to realize more minute structures and half-tones, which adds to the cost of manufacturing.
In another aspect, there are strong demands for smaller liquid crystal driving devices in liquid crystal display devices, to enhance portability, which adds the importance of reduction in the size of the liquid crystal driving device.
However, since as mentioned above, conventional technology requires greatly larger number of circuit arrangement elements for more minute structures and half-tones, the chip size grows when the liquid crystal driving device is fabricated as an integrated circuit, which renders it difficult to reduce the size.
The present invention has an object to offer a DA converter that, despite a possible increase in the number of voltages required, restrains large increases in the number of circuit arrangement elements (resistor elements, switches, etc.) and hence restrains increases in manufacturing cost, and that can be built in a more compact size.
The present invention has another object to offer a liquid crystal driving device that, despite a display of more colors and half-tones, restrains large increases in the number of circuit arrangement elements and hence restrains increases in manufacturing cost, and that can be built in a more compact size.
In order to achieve the object, a DA converter in accordance with the present invention is for converting N-bit digital signals to analogue signals by generating mutually different standard voltages and providing 2N output voltages based on the standard voltages according to the digital signals, and is characterized in that
the DA converter includes:
a standard voltage generating circuit for generating 2(Nxe2x88x921)+1 mutually different standard voltages,
selector circuit for: storing standard voltage pairs in advance so that none of the standard voltage pairs produces an identical mean value and also that each of the digital signals corresponds to one of the standard voltage pairs; upon reception of one of the digital signals, selecting one of the standard voltage pairs corresponding to the received digital signal; and providing the standard voltages constituting the selected standard voltage pair for output; and
an output circuit for receiving the output standard voltages of the selector circuit, and providing as an output voltage a mean value of the received standard voltages.
The DA converter converts digital signals to analogue signals by, for example, generating mutually different standard voltages through division by means of resistor elements and turning on/off switches according to N-bit digital signals to provide an 2N output voltages based on the standard voltages.
In such an arrangement, the standard voltage generating circuit generates 2(Nxe2x88x921)+1 different standard voltages. The selector circuit stores in advance standard voltage pairs so that each of the digital signals has a corresponding standard voltage pair. Here, no standard voltage pairs produce the same mean value. Upon reception of an input digital signal, the selector circuit selects one of the standard voltage pairs which corresponds to the input digital signal by, for example, turning on/off switches according to the digital signal and provides the standard voltages constituting the selected pair for output. The standard voltages provided for output by the selector circuit are supplied to an output circuit which provides an output voltage having a mean value of the input standard voltages.
In this manner, the voltages generated by the standard voltage generating circuit are subjected to predetermined calculations to generate voltages that are not generated by the standard voltage generating circuit. As a result, those voltages that are not generated by the standard voltage generating circuit become available for output, as well as the voltages generated by the standard voltage generating circuit.
In this manner, the standard voltage generating circuit may generate less voltages than actually required; therefore, the number of elements, for example, resistor elements, in the standard voltage generating circuit can be greatly reduced in comparison with conventional technology. Further, since the number of voltages generated by the standard voltage generating circuit is relatively small, the number of elements, for example, switches for turning on/off, in the selector circuit for selecting some of those voltages can be greatly reduced in comparison with conventional technology. Consequently, despite a possible increase in the number of voltages required, large increases in the number of circuit arrangement elements (resistor elements, switches, etc.) can be avoided and increases in manufacturing cost can be restrained, allowing the device to be built in a more compact size.
Further, in order to achieve the object, a liquid crystal driving device in accordance with the present invention is for converting display data from digital to analogue for output via a liquid crystal driving voltage output terminal to apply the DA converted data to a liquid crystal display element, and is characterized in that the DA conversion is done using a DA converter arranged in the above manner.
In this arrangement, output voltages are provided in a similar manner to the earlier arrangement. Therefore, similarly to the earlier arrangement, despite a possible increase in the number of voltages required in an attempt to display more colors and half-tones, the standard voltage generating circuit may generate less voltages than actually required; therefore, the number of elements, for example, resistor elements, in the standard voltage generating circuit can be greatly reduced in comparison with conventional technology. Moreover, similarly to the earlier arrangement, since the number of voltages generated by the standard voltage generating circuit is relatively small, the number of elements, for example, switches, in the selector circuit for selecting some of those voltages can be greatly reduced in comparison with conventional technology, which allows a great reduction in the size of the circuit in comparison with conventional technology. Consequently, the DA conversion using such a DA converter restrains large increases in the number of circuit arrangement elements in an attempt to display more colors and half-tones, thus restrains increases in manufacturing cost, and allows the device to be built in a more compact size.
Especially, a reduction in the size of the selector circuit is possible with every single liquid crystal drive voltage output terminal; accordingly, the use of smaller liquid crystal drive voltage output terminals amounts to great reductions in the circuit size when the entire liquid crystal display device is taken into account, enabling great reductions in chip size and manufacturing cost of the liquid crystal driving device when it is fabricated as an integrated circuit.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.